The present invention generally relates to communication systems. More specifically, the present invention relates to a method and apparatus for estimating frequency offset and timing offset in a communication system.
In a collaborative communication system, two Mobile stations (MSs) each having a transmit antenna can transmit signals to a Base Transceiver Station (BTS) on the same channel. Such a transmission is collaborative, since the signals transmitted from each of the transmit antennas of the two MSs, are spatially multiplexed through the utilization of the same tiles. The signal includes a plurality of tiles. In the collaborative Uplink (UL), Partial Usage of Sub-Carrier (PUSC) tile structure includes three Orthogonal Frequency Division Multiplexing (OFDM) symbols and four frequency sub-carriers according to IEEE 802.16e standard. Data sub-carriers of each of the two MSs are superimposed and pilot sub-carriers of each of the two MSs are placed on opposing corners of the tile, i.e. pilot symbols from each user are orthogonally allocated.
While transmitting the signal from each MS to the BTS a timing offset may occur between each MS and the BTS. A timing offset occurs due mismatch between a clock at a transmitter of a MS and a clock at a BTS. Similarly, a frequency offset may occur between each MS and the BTS due to a Doppler and a local oscillator mismatch between the MS and BTS. Additionally, timing offsets and frequency offsets may occur due to temperature changes, aging and other factors.
Timing offset and frequency offset manifest in channel estimation errors, thereby inducing a loss in Signal-to-Noise Ratio (SNR) and can lead to Inter-Carrier Interference (ICI). A significant loss in SNR and the presence of ICI contribute to degradation in the link performance.